Today in AP Physics 2, students refined their conceptual understanding of buoyancy and Archimedes Principle through the use of a hydrometer to measure the specific gravity of three different liquids. In addition, this was our first opportunity to use Crocodoc to annotate a PDF that I distributed for the activity. One issue that we encountered is that we couldn’t figure out how to select a drawn annotation on the Chromebook so it could be deleted. Clicking on the annotation in Safari on my Mac worked fine. While students working on the Chromebooks at their desks works fine, it was a bit awkward to carry them from station to station as opposed to carrying a notebook.
This year I added a new question to the activity handout:
In which liquid does the hydrometer experience the greatest buoyant force? Justify your answer.
Of eleven groups, only two had the correct answer and only one could justify it with strong evidence. This question, and the resulting discussion, is what makes this activity so valuable. In one class, as a student justified his group’s incorrect answer, I could see the realization once he heard himself explain that the weight of the hydrometers is the same in all liquids. I love how we keep returning to the balanced force particle model as we work our way through fluid statics.